1. Field of the Invention
This invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus which has a level shifting function or device.
2. Description of the Prior Art
Generally, a liquid crystal display apparatus displays pictures for video signals by controlling the light transmissivity of a liquid crystal. Generally, the liquid crystal display apparatus includes a liquid crystal panel arranged in a matrix type, driving integrated circuits (D-ICs) for driving the liquid crystal matrix, and a programmable logic device (PLD) for generating various control signals in the shape of pulse to control the D-ICs. The liquid crystal display apparatus further includes a level shifting device, depending upon whether either amorphous silicon thin film transistors (TFTs) or poly silicon TFTs were arranged to the liquid crystal panel to serve as a switch, for selectively switching data signals applied to liquid crystal cells. The level shifting device is needed since a driving voltage level of amorphous silicon TFTs is different from poly silicon TFTs.
As shown in FIG. 1, a liquid crystal display apparatus employing a liquid crystal panel 10, in which amorphous silicon TFTs are arranged together with liquid crystal cells, includes D-ICs 12 and a PLD 14. The PLD 14 generates timing control signals for controlling the operations of the D-ICs 12, and transfers video data from the outside thereof to the D-ICs 12 disposed on the edges of the liquid crystal panel 10. The timing control signals outputted from the PLD 14 and the video data have a swing width of 5.0 V or 3.3 V that is equal to a driving voltage of amorphous silicon TFT. The D-ICs 12 switch the amorphous silicon TFTs on the liquid crystal panel 10 and applies signal voltages corresponding the video data to the liquid crystal cells, thereby displaying a picture corresponding to the video data.
As shown in FIG. 2, a liquid crystal display apparatus employing a liquid crystal panel 20, in which poly silicon TFTs having an operating voltage level above 20 V higher than that of the amorphous silicon TFT are arranged together with liquid crystal cells, further includes a level shifting device 22 in addition to the D-ICs 12 and the PLD 14. The level shifting device 22 is connected between the PLD 14 and the D-ICs 12 to shift voltage levels of both timing control signals to be transferred from the PLD 14 to the D-ICs 12 and video data such that swing widths of the timing control signals and the video data increase from 5.0 V or 3.3 V to 20 V. Using the timing control signals and the shifted video data level, the D-ICs 12 drives the liquid crystal panel 20 comprising the poly silicon TFTs.
In order to shift the voltage levels of the timing control signals and the video data simultaneously, the level shifting apparatus comprises a number of level shifters.
Usually, amplifiers as shown in FIG. 3 are used for the number of level shifters included in the level shifting apparatus. The amplifier includes an operational amplifier 30 for receiving an input signal in a shape of pulse having a swing width V1 at the non-inverting terminal (+) thereof, a first resistor R1 connected between the inverting terminal (-) of the operational amplifier 30 and a ground GND, and a second resistor R2 connected between the inverting terminal (-) and the output terminal of the operational amplifier 30.
The amplifier 30 is driven with a high level voltage V+ and a low level voltage V- and amplifies the input signal on the non-inverting terminal (+) thereof by a voltage amplification ratio Av corresponding to a resistance ratio of the first resistors R1 to the second resistor R2 plus 1, i.e., Av=1+R1/R2. As a result, a pulse signal having a swing width corresponding to a difference between the high level voltage V+ and the low level voltage V- is outputted at the output terminal of the operational amplifier 30. If the difference voltage between the high level voltage V+ and the low level voltage V- is 20 V, then a pulse signal having a swing width of 20 V is outputted at the output terminal of the operational amplifier 30.
Alternatively, a comparator as shown in FIG. 4 may be used for the level shifters included in the level shifting apparatus. The comparator 40 has an inverting terminal (-) for receiving a reference voltage and a non-inverting terminal (+) for receiving an input signal in a shape of pulse having a swing width of V1. The reference voltage is set to have a voltage lower than the highermost voltage level and higher than the lowermost voltage level.
Accordingly, a pulse signal corresponding to a difference voltage between the high level voltage V+ and the low level voltage V- emerges at the output terminal of the comparator 40. If the voltage difference between the high level voltage V+ and the low level voltage V- is 20 V, then a pulse signal having a swing width of 20 V is outputted at the output terminal of the comparator 40.
The above amplifier and comparator have a complicate circuit configuration because they require a relatively large number of circuit devices. This results in a complication in a circuit configuration of the level shifting apparatus as well as having a difficulty in simplifying the liquid crystal display apparatus. Also, the amplifier and the comparator used for the level shifter waste a relatively large amount power and have a slow response speed.